National and regional under-5 mortality rate by economic status for low-income and middle-income countries: a systematic assessment

Background The progress to achieve the fourth Millennium Development Goal in reducing mortality rate in children younger than 5 years since 1990 has been remarkable. However, work remains to be done in the Sustainable Development Goal era. Estimates of under-5 mortality rates at the national level can hide disparities within countries. We assessed disparities in under-5 mortality rates by household economic status in low-income and middle-income countries (LMICs). Method We estimated country-year-specific under-5 mortality rates by wealth quintile on the basis of household wealth indices for 137 LMICs from 1990 to 2016, using a Bayesian statistical model. We estimated the association between quintile-specific and national-level under-5 mortality rates. We assessed the levels and trends of absolute and relative disparity in under-5 mortality rate between the poorest and richest quintiles, and among all quintiles. Findings In 2016, for all LMICs (excluding China), the aggregated under-5 mortality rate was 64.6 (90% uncertainty interval [UI] 61.1-70.1) deaths per 1000 livebirths in the poorest households (first quintile), 31.3 (29.5-34.2) deaths per 1000 livebirths in the richest households (fifth quintile), and in between those outcomes for the middle quintiles. Between 1990 and 2016, the largest absolute decline in under-5 mortality rate occurred in the two poorest quintiles: 77.6 (90% UI 71.2-82.6) deaths per 1000 livebirths in the poorest quintile and 77.9 (72.0-82.2) deaths per 1000 livebirths in the second poorest quintile. The difference in under-5 mortality rate between the poorest and richest quintiles decreased significantly by 38.8 (90% UI 32.9-43.8) deaths per 1000 livebirths between 1990 and 2016. The poorest to richest under-5 mortality rate ratio, however, remained similar (2.03 [90% UI 1.94-2.11] in 1990, 1.99 [1.91-2.08] in 2000, and 2.06 [1.92-2.20] in 2016). During 1990-2016, around half of the total under-5 deaths occurred in the poorest two quintiles (48.5% in 1990 and 2000, 49.5% in 2016) and less than a third were in the richest two quintiles (30.4% in 1990, 30.5% in 2000, 29.9% in 2016). For all regions, differences in the under-5 mortality rate between the first and fifth quintiles decreased significantly, ranging from 20.6 (90% UI 15.9-25.1) deaths per 1000 livebirths in eastern Europe and central Asia to 59.5 (48.5-70.4) deaths per 1000 livebirths in south Asia. In 2016, the ratios of under-5 mortality rate in the first quintile to under-5 mortality rate in the fifth quintile were significantly above 2.00 in two regions, with 2.49 (90% UI 2.15-2.87) in east Asia and Pacific (excluding China) and 2.41 (2.05-2.80) in south Asia. Eastern and southern Africa had the smallest ratio in 2016 at 1.62 (90% UI 1.48-1.76). Our model suggested that the expected ratio of under-5 mortality rate in the first quintile to under-5 mortality rate in the fifth quintile increases as national-level under-5 mortality rate decreases. Interpretation For all LMICs (excluding China) combined, the absolute disparities in under-5 mortality rate between the poorest and richest households have narrowed significantly since 1990, whereas the relative differences have remained stable. To further narrow the rich-and-poor gap in under-5 mortality rate on the relative scale, targeted interventions that focus on the poorest populations are needed. Copyright (C) The Author(s). Published by Elsevier Ltd.